Form, Appearance
If you put a roof over a valley it will change its form. Its appearance will alter completely!
Picture a valley in your mind. Now put a roof on it. Hmm. It isn't shaped like a valley anymore.
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Its new appearance looks familiar to you, like it's a big yogurt (よう) cup! A valley with a roof on it looks exactly like a yogurt cup. Huh!
Now imagine this taking shape before your eyes and becoming actual yogurt! You grab a giant spoon and take a bite.